PTFE is a material with particular properties, such as low friction, low adhesion to other materials, low surface tension, high resistance to chemicals etc. When PTFE is heated to temperatures at which the joining can be carried out, a substantial increase in volume occurs. Joining of parts of PTFE by means of heating is usually referred to as welding. The parts often have a particular shape and particular tolerances, which are desirable to maintain after welding. In order to obtain a satisfactory welding, the PTFE material must be heated to temperatures close to the melting point, and normally certain precautions are required to avoid permanent deformations, which will have to be machined further to obtain desired dimensions.
DE 23 11 096 describes a method of joining PTFE parts by the addition, during welding, of an intermediate layer of a meltable binder containing perfluoroalkoxypolymer (PFA).
In the industrial configuration, the PTFE parts are generally welded in a straight joint, i.e. the parts' joint surfaces to be welded are positioned directly opposite each other, and heating can be used by means of heating surfaces, which are in contact with the PTFE parts as described in U.S. Pat. No. 4,211,594. In case of rod-shaped parts, the heating surfaces also cover the sides of the parts at some distance from the welding surfaces of the parts, and the heating surfaces assume the form of a closed ring, which fully or partially surrounds the welding zone. Thus, the possibilities of making the parts movable with respect to each other, as described in the patent, are limited. As a result, the heating ring is usually constructed with a certain oversize so that contact is obtained due to thermal expansion, as described in U.S. Pat. No. 4,701,291. Accordingly, the heating ring will also serve as a means of ensuring correct mutual positioning of the parts to be joined.
The binder can for instance be added to the welding surfaces as a separate component in the form of a film, e.g. at the same time as the parts are placed in the welding tool, and binding or welding is obtained by heating to a temperature, which is usually above the melting temperature of PTFE, typically between 330 and 370° C., at the same time as a certain contact pressure, typically between 0.1 and 2 MPa, is exerted. As a result of the change in volume due to the heating and the mechanical influence of the soft, heated PTFE material, undesirable, permanent changes to the dimensions around the welding zone often occur. This constitutes a problem when finished profiles are to be joined, and will usually require further mechanical treatment of the area close to the welding to obtain the desired dimensions.